(CBS/AP) Two U.S. agents shot in Mexico, one fatally, were ambushed, CBS News has learned. Sources say the Immigration and Customs Enforcement agents, driving a Chevy Suburban with diplomatic plates, were followed Tuesday by two vehicles on a toll highway headed from Mexico City to Monterrey, before as many as ten gunmen attacked.

Agent Jaime Zapata was killed.

At one point, law enforcement sources say, one vehicle passed the agent's Suburban and cut it off. The Suburban was trapped between the two cars loaded with up to ten gunmen, who surrounded the Americans, although it is unclear if the gunmen knew there were U.S. agents in the car.

One of the agents rolled down his window and both agents were shot. He rolled the window back up and other gunmen opened fire on the armored vehicle with high powered weapons, sources told CBS News.

The gunmen fled and the agents were able to call for help on a cell phone. ICE agent Victor Avila suffered gunshot wounds to the leg but survived the ambush. He has been returned to the United States.